after some googling, this came up (a response in a Reddit topic):
Spoilers ahead.
Five kids who tangentially know each other (two cousins Jake and Rachel, their best friends Marco and Cassie, and a stray bully magnet named Tobias who's awkwardly following Jake around) end up walking home from the mall together, cut through an abandoned construction site, and meet an alien named Elfangor. He explains that mind-control slugs called Yeerks are taking over the world, has them touch a blue cube that gives them the ability to turn into any animals they can touch, then has them hide. Yeerk controlled aliens land, one of which is another Andalite who turns into a giant monster and eats Elfangor. That's good ol' Visser Three. The freaked out kids run like hell and escape.
Over the next few days they learn how to morph and learn that their assistant principle Chapman and Jake's big brother Tom are both controllers, and they're recruiting more by way of a club called the Sharing. They decide they can't just ignore what's happening, so they find a secret entrance to the Yeerk Pool that's buried under a large part of the town and sneak in. Things Go Wrong, and as they escape they have to leave Tobias behind. He escapes, but before he does he stays in hawk morph too long and becomes trapped that way. He moves into Jake's attic, sleeps over at Rachel's sometimes, and lives on leftovers for a month or so.
Meanwhile they go on a few more information gathering missions and even manage to blow up a large ship the Yeerks use for gathering air and water for their orbiting mothership. Along the way Tobias struggles with being a hawk and nearly kills himself, but he comes to terms with it and takes over a meadow behind Cassie's farm.
Cassie and Tobias reveal that they've been getting weird dreams, and then they see a news program about washed up wreckage from what they realize was another Andalite ship. They conclude that the dreams are a distress signal, morph dolphins, and rescue Aximili from the dome of the Andalite mothership under the ocean. He joins the Animorphs and moves into the woods behind Cassie's farm, and he also creates a human morph by combining DNA from everybody except Tobias.
They have several more adventures over the following months, including Jake being made a temporary controller, meeting the trickster-god Ellimist, blowing up the Kandrona machine that the Yeerks depended on to feed while planet-side, engaging in biological warfare by way of oatmeal, visiting the Yeerk Pool Ship in orbit, learning that Marco's "dead" mom is Visser One, discovering that their classmate Eric is a super advanced dog-robot called a Chee, and a whole mess of near death experiences.
Around half a year in, Tobias helps some Hork-Bajir (big blade-covered lizard aliens) escape from the Yeerks to hide in a valley the Ellimist has enchanted to keep hikers and Yeerks away. As a reward the Ellimist restores Tobias's ability to morph and lets him acquire his old human body, but he leaves Tobias's default form that of a hawk.
They have a bunch more adventures over the next several months, including meeting a cannibal Yeerk, an accidental trip to an alien world inhabited by psychic amphibians, and Cassie very nearly being trapped forever as a caterpillar/butterfly to befriend a Yeerk named Aftran.
Then they meet a kid named David, who has found the morphing cube. Unfortunately the Yeerks also learn of him and smash his house up before the Animorphs can get the cube away from the little dipshit. They decide to rope him in and give him the ability to morph, but he turns out to be a murderous asshole, so they leave him trapped as a rat and get on with life.
A little bit later the Yeerks discover that Tobias's dead father was actually Elfangor, so Visser Three pretends to be his cousin to sucker him in. Tobias decides that's dumb so he pretends to be a delinquent to discourage the Yeerks from giving a shit, when he's not too busy riding speeding cop cars and helicopters. Meanwhile we also learn that those two Hork-Bajir that Tobias rescued have been busy; they've rescued a bunch more Hork-Bajir on their own and have also had a genius kid who they named Toby after Tobias.
They have another year or two of zany adventures including Cassie doing brain surgery on Ax with the assistance of Aftran, Jake and Cassie using the power of love to turn the murder-hobo Howlers who serve the Ellimist's evil rival Crayak cuddly, a trip to the Hork-Bajir homeworld along with Toby to make the ghost of an Andalite turned Hork-Bajir happy, and Marco coming to terms with getting a stepmom.
Then things heat up. Visser Three gets promoted to Visser One and the Yeerks start targeting higher profile people like the governor and generally getting ready to significantly expand their invasion. Meanwhile the Andalites finally get off their asses and start heading over to Earth planning to defeat the Yeerks by very probably committing genocide and wiping out humanity, as they once attempted and partially succeeded at with the Hork-Bajir.
The Animorphs decide to up their own game in response. They rescue Marco's mom from the former Visser One, find and rescue Tobias's blind amnesiac formerly-space-adventuring mom from Yeerks who also tracked her down, and then move them into hiding along with Cassie and Rachel's families and Marco's dad (Marco's stepmom and Jake's family are unfortunately left to the Yeerks). They all set up camp in the Hork Bajir Valley, which is now a bustling community. They finally get over the David debacle and recruit more Animorphs, and they notify the military about the Yeerk threat. Then they blow up the Yeerk Pool, wrecking a big part of the city in the process.
The Yeerks respond by abandoning all subtlety and bringing the pool ship down from orbit to hover over the crater. Then the Animorphs sneak aboard the pool ship with some help from Tom's Yeerk (who's rebelling, but not joining the peace movement like Aftran) and a big suicidal distraction from the military and their new Animorphs, capture Visser One, and secure the pool ship just in time to claim the win and prevent the Andalite military from fucking everyone over. Along the way Rachel goes on a suicide mission to eliminate Tom's Yeerk; she and Tom both die in the process, and Tom's rebel Yeerk faction escapes in the Blade Ship.
Everybody else lives unhappily ever after, aside from Cassie who is doing alright. The Yeerks are all allowed one use of the morphing tech to trap themselves as snakes and they're moved to a preserve in the jungle. The Chee go back into hiding. Marco goes on TV. Ax becomes an important Andalite. Earth becomes a tourist destination, because cinnamon buns. Zuh. Bun-zuh. And Jake and Tobias mope around until Ax goes missing, then those two, Marco, and some military recruits Jake was training in the morphing tech take a ship and go adventuring. Ax is found aboard the stolen Blade Ship borged by an unknown entity, and they ram it. The end.
Edited to fix errors. Visser Three masqueraded as Tobias's cousin, not aunt, and this was no more than a year in (it was Tobias's first birthday as a hawk).