Twelve-year-old sweat-averse Joey is psyched for his summer of relaxation and perfecting his butt-shaped indentation on the couch. Sure, he wishes that his best friend, Xander, wasn’t always busy with his new cross-country team, but Joey is certain that Xander will still have plenty of time for the things they love: watching Italian soccer, making homemade pasta with Joey’s Nonna, and debating the merits of their favorite superheroes.

But Joey's summer plans are upended when his perfect older cousin, Leo, comes to stay at his house for the summer. Leo is the kind of guy who runs marathons and jumps off cliffs. Hanging out with Leo is dead last on Joey's list of Ways I’d Like to Spend a Day, partly because the two cousins have a tense history that goes back to the death of their Nonno. But now, Leo’s training to climb the highest mountain in California, and he wants Joey to do it with him. The plan is nuts—Joey usually avoids climbing stairs, so seriously, a mountain? But somehow, he’s roped into hiking with Leo anyway, which is how he accidentally learns Leo’s secret: His cousin has young-onset multiple sclerosis. So newsflash: This climb will most likely be his cousin's final adventure, and he needs Joey's help to do it.

With Xander spending more time with his new running friends, Joey’s mom on his back about being a good family member, and Leo’s disease slowly progressing, Joey feels the pressure building. And as the hikes get harder, Joey must confront not only his own physical limitations, but also a friendship he thought was unbreakable, his feelings for a girl who’s way out of his league, and a changing relationship with his cousin that will test his strength—and courage—in ways for which he’s not prepared.