Magnum, P.I. (1980)

After being expelled from a number of expensive schools, R.J., Robin's rich, careless, teenage nephew, arrives at the estate to be reformed and educated by Higgins. R.J., however, has other ideas and decides to impersonate Thomas after being impressed by his adventures. Hired by a woman to find the only witness to her husband's murder, R.J. manages to get Thomas and everyone else involved in the case targeted for death by the men who killed the woman's husband.