Pete [George Peppard] is a former advertising executive living a Beatnik–Bohemian life, part of a New York City loft commune where creativity often borders on cynicism. The group is defined by a sense of aimlessness and melancholy, and one resident even lives in a burlap sack with only her bare feet showing.
One day, a wayward toucan arrives at the loft, having stowed away on a Greek banana boat from South America. The bird carries a unique and highly contagious virus that triggers intense feelings of giddiness, happiness, and kindness in anyone it infects.
Pete is among the first to catch the virus, and in a sudden burst of euphoria he finds a new purpose. His girlfriend Liz Mary Tyler Moore is initially horrified by his change, and when she learns from nearby police about the bird’s virus she tries to warn him. But Pete has already shaved his beard and proposes marriage and a return to conventional life.
In a bid to spread the contagion, he adopts a disguise as the nihilist German philosopher leader of a doomsday cult popular in the commune and attempts to pass the virus through close facial contact. In this deception, he persuades Liz to allow a kiss, but his true identity is soon revealed.
The now upbeat collective decide to keep the toucan and nickname it Amigo. They set out to infect as many people as possible across New York City, blending into ordinary life in classic fashion. Liz remains physically immune, yet the warmth she feels from her friends draws her toward kindness herself. When authorities close in on the bird, Pete and Liz manage to spirit him away by hiding the bird in Liz’s dress and pretending she is pregnant, though the ruse becomes tangled when “nice” police take the couple to a hospital to give birth.
The virus rockets through the city: rude telephone clerks become courteous, customers and colleagues treat each other with consideration, and even those who are immune or who normally resist the contagion swing toward gentler behavior. Pete returns to his advertising job, insisting that all ads be honest; he’s initially fired, then rehired with a raise when his peers also fall prey to the virus and the company recognizes his unorthodox yet effective vision.
Facing a threat to the economic lifeblood of New York City, government leaders deploy a plan to end the outbreak. J. Gardner Monroe [Dom DeLuise] arrives wearing a space helmet to supervise the intervention. After several attempts, the toucan is captured and a cure is found. The vaccine is dispersed through gasoline and industrial exhaust fumes, and those cured revert to their previous, gruffer selves, while those who remained immune continue to act nicely.
Pete returns to the loft with a renewed perspective, though Liz confesses she cannot live with this constant “niceness” and admits she preferred him when he was “sick.” She visits a zoo to say goodbye to Amigo, who is now caged. Pete shows up, and the bird escapes its cage, leading to a final chase. Liz pretends to be pregnant again to retrieve Amigo, and together they escape from the zoo.
