Meanwhile, the F.B.I. was turning up the heat, issuing a bulletin to law-enforcement agencies throughout the Northeast. Picasso stolen from Logan airport. Be on the lookout.

Days later, the ice cream king of Waterville arrived in Medford with his wife, Ann, a new trench coat, and a plan. He rubbed the painting’s packaging and crate with Vaseline, for reasons that evaded his son. He attached the handwritten note. He donned the trench coat, a brimmed hat, and gloves. Go time.

Three years after this escapade, Whitcomb Rummel would die, suddenly, at 63; in his honor, his restaurant would stay closed until the evening ice cream rush. His son Bill would spend the next 30 years with Emery, rising to regional manager before retiring to South Carolina and dying, at 71, in 2015.

But on this April Fools’ Day in Boston, 1969, father and son were sharing an unforgettable moment: loading a purloined Picasso into a Chevy Impala.

Bill Rummel, wearing a black watch cap and sunglasses, drove them into Boston and, at his father’s direction, parked on Huntington Avenue. His father got out and carried the crate a few car lengths ahead.

The elder Rummel loaded the painting into a taxi, handed the driver a $20 bill and told him to deliver the package to the Museum of Fine Arts, just down the avenue. He returned to his son’s car and, on the drive back to Medford, tossed the coat, hat and gloves in separate garbage cans.