Ryland Grace

Ryland Grace is a scientist and professor specializing in biology and applied science. Grace is very intelligent and creative, especially good at solving problems using scientific reasoning. Grace is sarcastic; he frequently makes jokes and ironic comments, even in absurdly dangerous situations.
Grace is fearful in a very human way (being quite humanized in the story, both in the book and the film); he doesn't start out as a fearless hero. He's afraid of dying and questions his own decisions quite a bit.
Grace is curious when he encounters something he doesn't understand; his reaction is to try to figure out how it works. Grace is empathetic, a trait that becomes increasingly important as he gets to know Rocky. He's somewhat clumsy, making mistakes, taking bad decisions, and sometimes basically panicking.
—Rocky: Rocky is a member of an alien species that comes from the planet Erid. Rocky's species evolved on Erid, a planet with environmental conditions very different from Earth. Five main limbs, with structures that he uses to manipulate objects. Eridans do not have vision like humans. They perceive their environment mainly through sound and vibrations. They have an extremely resistant body. They can withstand conditions that would be extremely dangerous for humans. Eridans are adapted to an environment very different from Earth.
And one of the most interesting things:
Rocky doesn't "see" Grace.
For him, Grace is a combination of sounds, vibrations, and other sensory information. This influences practically everything about them. In their communication: While humans primarily use vocal and visual language, the Eridanos utilize sounds and vibrations.
This makes communication between Grace and Rocky extremely difficult at first.
They literally need to build a common language.
And it's one of the most beautiful parts of the story because the two of them start out without even knowing if the other is intelligent...
...and end up developing a deep friendship. Rocky is:
Kind, patient, extremely loyal, and incredibly hardworking.
He also has his own sense of humor, although it takes Grace a while to understand it.
And there is a very important characteristic about it:
Rocky is extremely brave.
But not because I'm not afraid.
He knows his mission could cost him his life.
Even so, he continues working to save Erid.
And when he meets Grace, his concern is no longer just for his own people.
He begins to genuinely care about humanity as well.
—Rocky's species— the Eridanos: Intelligence
The Eridani are extremely intelligent.
But the way their society thinks is not necessarily the same as the human way.
Rocky, for example, demonstrates a combination of:
Engineering + mathematics + curiosity + sense of responsibility.
He is especially skilled in construction and engineering.
And there is a very interesting difference between him and Grace:
Grace thinks like a biologist.
Rocky thinks like an engineer.

(The lore is pretty well explained, so some details were left out because otherwise it would be the longest text I've ever written in my life ;—;)
At the beginning of the story: The problem: the Sun is dying.
The story begins with Earth facing an astronomical catastrophe. A type of microscopic organism called Astrophage is spreading through space and consuming the energy of stars.

The problem is that the Sun is losing energy rapidly. If this continues, the Earth will get colder and colder, and eventually humanity will die out.

But there is an important discovery: Astrophage is not simply a pest. It possesses characteristics that can be exploited as a source of energy and propulsion.

Scientists have also discovered that other stars are experiencing the same phenomenon.

This raises the possibility that some alien civilization is facing the same problem.

The protagonist is Ryland Grace, a professor and scientist who initially did not want to participate in a space mission.

He ends up being chosen for an extremely desperate mission: to travel to another star system to discover how to stop the process that is causing the stars to lose energy.

The creepy detail?

The trip is one-way only.

Because of the enormous distance and technological limitations, Grace knows she will probably never return home.

It is placed aboard a spacecraft along with vast amounts of equipment and biofuel based on the Astrophage.

Grace wakes up inside the spaceship...

no memory.

He doesn't know who he is, where he is, why he's there, or what his mission is exactly.

Gradually, using clues scattered throughout the spaceship, he begins to reconstruct his own memories.

It's practically a gigantic scientific puzzle.

He discovers he is traveling to Tau Ceti, where there is evidence that something may be helping that star resist the problem.

THE ALIEN

Grace discovers that she is not alone.

There is another spacecraft traveling to Tau Ceti.

Inside it is an alien.

And he is... a "stone" creature resembling a spider but with 5 legs, extremely intelligent and more advanced than humans.

His name, roughly translated, is Rocky.

Grace is initially terrified.
But Rocky isn't hostile.

He is also trying to save his planet.

And the two realize that their species face exactly the same problem.

Grace and Rocky start working together.

The coolest thing is that they don't even share the same senses.

Rocky perceives the world primarily through vibrations and sounds, while Grace relies on sight and other human senses.

So they literally need to figure out how to communicate.

And this friendship ends up becoming the heart of the story. They go from strangers to friends who would do anything for each other.

They discover the secret of the Astrophage.
Their investigation reveals that Astrophage functions in an extremely peculiar way.

It absorbs stellar energy and can store it, allowing it to be used as a kind of absurdly efficient fuel.

This explains how spacecraft are able to travel between star systems.

But they still need to figure out how to stop Astrophage from continuing to steal energy from the stars.

To do this, they need to investigate the organisms that control or destroy Astrophage.

And that's where the Taumoeba come in.

These organisms are capable of consuming Astrophage.

The problem is that they need to figure out how to cultivate and control these creatures.

Grace and Rocky conduct a series of experiments.

They discover that there is a specific temperature at which they can work with certain organisms, and that the environmental conditions of each planet complicate everything.

The fun part about the book is that a large part of the story is basically:

"What if we try this?"

"It went wrong."

"What if we try that?"

"That also went wrong."

"SCIENCE!!!"

Until they finally find a way to produce a substance capable of fighting Astrophage.

While all this is happening, Grace gradually begins to remember the past.

And he discovers something important:

He hadn't simply been chosen for the mission.

In fact, he initially refused to participate and was forced into the mission.

Grace was afraid of dying and wanted to continue living her life.

But humanity's situation became so desperate that he ended up being involved in the mission.

And there's an even more personal reason for him being there.

Grace discovers that the other members of the original mission were Yáo and Stratt, and that the plan involved sending humans to other systems to try and find a solution.

Grace believed he had been chosen because he was an exceptional scientist.

But the reality was more complicated.

He ended up being put on the mission because his skills were needed.

And, during the process, he came to accept that he might need to sacrifice his own life for humanity.

Grace and Rocky finally manage to develop a solution using organisms that can fight Astrophage.

But there is a huge problem:

They need to transport the solution back to the source systems.

And Grace discovers that Rocky also needs to return to his planet.

They both have a choice.

Grace can return to Earth.

Rocky can return to his world.

Or one of them can help the other.

Grace discovers that Rocky is in danger because of the new Astrophage predator that would eat all of Rocky's Astrophage, leaving him vulnerable to radiation and eventually causing him to die slowly on his own.
So he decides to abandon his own return to save Rocky.
He changes course and goes after his friend.
It's one of the most important moments in history because it shows how much Grace has changed.
Initially, he was someone who desperately wanted to avoid dying.
Now he is willing to risk everything to save another person, or in this case, another being.
The ending: Grace manages to save Rocky.
But that means he can't return to Earth.
He ends up staying on Rocky's planet, where the Eridanos created a dome for human survival. Delico's Nursery
Grace moves there and helps develop the Eridanos community.
And, most importantly, he discovers that the mission was successful.
The solution they developed works.
The stars can recover.
Humanity is saved.
And Rocky's planet too.