A human-interest story crossed with tabloid elements of superstition, sympathy and terror, “Turtle Boy” (8 p.m. Sunday, National Geographic) recalls a young boy beset with a medical affliction that is straight out of science fiction or forest legend.

The son of poor farmers in rural Colombia, 6-year-old Didier Montalvo was born with congenital melanocytic nevus, a rare condition characterized by an abundance of moles on the body. Only in Didier’s case, they gathered to become one giant mole, encasing his back and parts of his chest like a giant tortoise shell. Neighboring children taunted him as “Turtle Boy.” Worse, his mother’s neighbors blamed her for his condition, whispering that she defied warnings and looked up during a rare eclipse of the sun.

After years of suffering, Didier’s story gained the attention of a Colombian television personality who turned the boy into a national celebrity and object of pity, soliciting donations that enabled Didier to travel to Bogota for a delicate and expensive procedure. Didier’s story also caught the attention of two London surgeons who are leading experts on the rare condition and wanted to treat the most extreme case on record.

As they explained, removing the giant mole was not merely for the sake of comfort or appearance. In most cases, these moles can become malignant and cause fatal cancers in their young hosts.

• “Inspector Lewis” enters its fifth season on “Masterpiece Mystery!” (8 p.m. Sunday, PBS). Oxford University and its bucolic surroundings are the backdrop for murders macabre and bizarre. Tonight: An obsessive professor is found slain and buried in a fashion that suggests a ritual killing.

Sunday’s other highlights

• Web therapy on “Secret Millionaire” (7 p.m., ABC).

• A fashion-forward woman requires the help of “Extreme Makeover: Weight Loss Edition” (8 p.m., ABC).

• In search of Russell on “True Blood” (8 p.m., HBO).

• Will apologizes for some of his past on-air outbursts on “The Newsroom” (9 p.m., HBO). Despite withering reviews, HBO has already announced a second season pickup for this series.

• In an awkward move for all concerned, Sean and Beverly decide to separate as a couple but collaborate on the set on “Episodes” (9:30 p.m., Showtime).