We follow perpetually-single Will as he attempts to woo women, while avoiding any real commitment. His life changes when a quirky, bohemian, vegan single mother, Fiona, and her 11-year-old son, Marcus, move in next door. Marcus is a misfit, and already being bullied at his new school, yet he and Will strike up an odd-couple friendship, under Fiona's disapproving watch.

Andy guilts Will into agreeing to babysit Marcus so Fiona can go on a job interview, but the situation gets complicated when they receive a last-minute invitation to a once-in-a-lifetime charity pool party hosted by rap star Lil Jon. Meanwhile, Fiona discovers that honesty isn't always the best policy when it comes to landing a job.

Will attempts a much-needed boy's night out with Andy, but complications arise, leaving Will to reach out to Marcus in hopes of salvaging the evening. When Andy later becomes available, he attempts to track down Will, but instead winds up bonding over the joys of parenting with Fiona. Meanwhile, Andy and his reluctant wife Laurie solicit Will to be godfather to their son Jonah.

Marcus asks Will for advice on how he can get Hannah to invite him to her upcoming birthday party. When that proves unsuccessful, Fiona takes it to task with Joanne, Hannah's mother, who also happens to be the queen bee of a group of neighborhood moms that she is trying to win over.

When Marcus starts hinting that Will and Fiona should start dating each another, Will does everything in his power to set him straight. Meanwhile, Fiona gets advice from Dakota to get back into the dating scene, and with very little game, seizes the opportunity by asking out her plumber - with results she didn't quite anticipate.

With Fiona putting in extra hours at work, Marcus finally gets his own house key, but he quickly loses his mom's trust when Will has to rush him to the emergency room following an untimely incident with a knife. However, Marcus' misfortune has a silver lining for Will when he crosses paths with Dr. Samantha Lake, a beautiful and charming doctor who comes to Marcus' aid. Will is instantly smitten and stops at nothing in his attempts to win her over - even if it means bending the truth.

Will throws a poker night for his friends, including Andy, Crosby, TJ and Richard, but the event is hijacked by Fiona, who has no problem making herself at home with the guys. Meanwhile, Marcus goes on his first sleepover, but has a hard time staying the night when he finds the others aren't as nice to him as he had hoped.

Fiona volunteers to run a project for Marcus's class - a miniature society modeled after Dickensian London. Wanting to be King of the mock economy, Fiona instead selects him as the lowly slopmaster, much to his dismay, prompting him to figure out a clever yet questionable solution to turn things around. Meanwhile, Will takes Marcus out on a spending spree, thanks to his latest royalty check and then does his best to throw the perfect party to impress Sam.

Andy and Laurie host a game night and Will invites Sam, but things don't go exactly as planned when she arrives. When his friend TJ asks Fiona to join him for the festivities, Will goes to great lengths to discourage any sparks from flying. Meanwhile, Marcus is uneasy when he discovers the mother of an older mean girl at school will be babysitting him.

Marcus is elated when his father, Hugh Womple takes a break from his penguin research in Antarctica to make a surprise visit. While Fiona does her best to mask her tormented attraction, she can't help but notice how consumed Hugh is with his work and his indifference towards anything Marcus is interested in. At Fiona's urging, Will steps in and does his best to encourage some much needed father-son bonding. Meanwhile, Sam's demanding work schedule throws a wrench into their plans for romance.

Marcus is excited for his first ever big birthday party and Will agrees to help plan it. But complications arise when Sam asks Will to a gala at the hospital, creating a scheduling problem. To make matters even worse, Will and Andy discover that one of Marcus's popular classmates is having a birthday bash on the same day. Meanwhile, Fiona and Sam get into a disagreement, and Will finds himself stuck in the middle.

Will invites Sam to temporarily move in with him when she is forced to evacuate her apartment. However, when Andy warns Will that his home will soon be transformed into her place, similar to what had transpired with him and Annie, Will starts to panic. Meanwhile, Fiona misreads a conversation and is under the false impression that Sam is pregnant and Will and Marcus build a treehouse together.

Will is completely heartbroken when Sam receives a job opportunity she can't refuse in New York, prompting Fiona and Marcus to try their best to comfort their friend. With her absence weighing on his mind, Will turns to Andy for his sage advice in figuring out how to best move forward.