Write a decent script — not a masterpiece but one with enough misdirection to keep us guessing. Hire a compelling lead. Imagine your ideal number of episodes, then lop off two or three of them. No, it really isn’t difficult to make a better-than-average TV murder mystery. ITV’s Gone, now streaming on BritBox following a much-lauded run in the United Kingdom this spring, succeeds mightily on the basis of modest choices. The show stars David Morrissey, a classically trained actor who knows how to fill small screens. It produces half a dozen “aha!” occasions, then revokes each one on the way to a logical conclusion. Most importantly, it exits the stage before we’ve grown sick of it. Hand it to the Brits: They’ve made in six episodes a series we greedy Americans would have stretched to 10. Morrissey plays Michael Polly, an English headmaster who falls under suspicion after the disappearance of his wife, Sarah. As taciturn as Jane Eyre’s Mr. Rochester, Michael is seemingly incapable of rising to the emotional moment, insisting, for example, that he is still the man to coach the school’s rugby squad even as Sarah’s body turns up in a nearby woods. Attempting to fathom this coldness is Detective Sergeant Annie Cassidy (Eve Myles), an officer as empathetic as Michael is aloof. Quietly appalled by her suspect’s lack of feeling, Annie sets in motion an investigative ordeal that makes little distinction between Michael’s actual innocence or guilt.