Wendy Mass is a woman who appeared in Mr. Monk and the Red-Headed Stranger, having apparently heard country singer Willie Nelson killing Sonny Cross. However, this is a lie, as she was actually the one who killed Cross. Cross had been drunk-driving when he hit her parent's car, killing them and blinding her. Cross was sentenced, but only to two years in prison, which enraged Wendy. After slipping and suffering a concussion at a grocery store years later, her vision was restored in one of her eyes. She still pretended to be fully blind, so she could kill Cross without suspicion.
Mass soon heard which radio station Cross would be at and taped a sign to the door instructing him to enter the station through a side alley. Once Sonny got close to the dumpster she was hiding behind, Wendy shot cross once at point blank range, then shot him again after he ran a few feet to insure police did not suspect her, as no blind person could have ever made the shot. After Cross fell dead, Wendy wiped her fingerprints off the gun and was ready to flee. Unfortunately for her, Nelson came running down the alley and checked on Cross. Since she now couldn't run, she pretended to be a witness to Willie murdering Sonny.
She was eventually discovered when detective Adrian Monk determined she was not completely blind after instinctively reaching out to shake Stottlemeyer's hand, when she shouldn't have known he broke one of his arms falling off a ladder. To prove it, Adrian bailed a streaker named Justin out of jail and asked him to streak past her in a park. When she turned to confirm the sight she was seeing, it proved to Stottlemeyer, Disher, and Sharona that she could see. Wendy admitted to the crime, saying that killing Cross was truly justice. However, she expressed remorse that Willie was being arrested for her crime, and asked that the only leniency she be given in her sentence was that her cell had a window.