Magnus Carstens, an eminent glaciologist, recently had to change his life: he's become his nephew Jakob's guardian after the boy's parents died in a plane crash, he's taken a new job as a park ranger, and he's moved them to Mistletoe, Wyoming, so he can offer a stable home to Jakob while still contributing to science. There he meets Lance Rivera, a disillusioned physical therapist who is working in his sister's store to make a living after his lover took off with all their money. When the two men come together—literally under the mistletoe—both realize all they really want, despite adversity, is a family to love.

Lance Rivera and Magnus Carstens are settling into their new life as a family with Magnus' nephew Jakob. But Magnus' parents are not enthusiastic about his choice of life partner and try to separate Magnus and Lance while they vacation in Sweden with Jakob. The woman they think more suitable for Magnus is Sigrid, an ex-student who is his intellectual equal and has loved and pursued him for years. Magnus seems oblivious, but when an attractive offer from the University of Stockholm is added to the mix, Lance faces a crisis: What if Magnus chooses the new job and a life with Sigrid over the family he's started with Lance and Jakob?

Lance is on cloud nine. His lover Magnus has proposed to him, making Lance the happiest man on Earth, and they are planning to move their family to Sweden for Magnus' new job at Stockholm University. But when an old enemy resurfaces, followed by an unexpected new threat to their relationship, their new life in Europe with Magnus' nephew Jakob turns out to be very different from what they had hoped. Will Lance and Magnus' love be strong enough to make their Mistletoe Experiment a success?