Benedict Cumberbatch does it all in "Patrick Melrose," an acting exercise that only feels like a TV series after the premiere.

There’s a scene in the first episode of “Patrick Melrose” that comes to define its successes and failures all in one simple cut. Patrick, played with incredible energy and impeccable precision by an all-too-enthusiastic Benedict Cumberbatch, is sprawled out on his dealer’s couch and injects himself with heroin — the very drug he swears he’s done with, but spends a good chunk of the first hour obsessing over — when, suddenly, the camera swings back behind his head and then swings quickly back in the opposite direction, only to find him inside his New York hotel room instead of the apartment he was just sitting in.

It’s one of the more dramatic moves by director Edward Berger (who helmed episodes of “Deutschland 83” and “The Terror”), as the rapid shifts in direction and location convey the wild rush of this craven high. Cumberbatch stretches out his body, on the couch and the floor, to parallel the frame’s motion, and the result makes his tortured trip feel like a thrilling plunge into the deep end of the pool. Though nothing audiences haven’t seen before, especially in shows and movies about drug addicts, the shot clues the viewer into what’s been staring them in the face for 30 minutes:

“Patrick Melrose” isn’t a TV show; it’s an acting exercise — at least, for the first hour. Cumberbatch gets quite a workout, but it’s not until the third episode that his early efforts fully pay off. Once you get a little backstory and see the contrast provided in Cumberbatch’s more subdued version of Patrick, that’s when you can appreciate everything he brings to the table — and perhaps become invested beyond his one-man show.

But in the premiere, there’s a very slight narrative playing out: Patrick’s father has died and he’s been sent to the Big Apple to collect his remains. But that’s only fuel for his fiery downward spiral, as well as motivation for Cumberbatch’s regular mood swings. The few other characters briefly step in and out of the episode as if they’re imaginary; all of Patrick’s visitors are secondary to his craving, and his craving is an internal drive made external by Cumberbatch’s committed performance.

So totally is Cumberbatch relied upon to sustain viewers’ interest, the premiere may as well have been a one-act play. The “Sherlock” and “Captain Marvel” star could’ve stood in the middle of a rotating stage that shifts locations for him, changing as needed to convey where he is (or where he thinks he is) and with co-stars popping in to ground him in reality whenever he’s actually in it (or thinks he’s in it). After all, Patrick spends much of the time speaking to himself. He responds to the voices in his head and doesn’t always know if he’s talking out loud or not. His brain is fried on uppers to counter the downers and more downers to counter those uppers. He is an addict — a glorious, spinning pinwheel of an addict with sparklers shooting out of each end.

That Cumberbatch has the output necessary for such a grandiose and unceasing performance is admirable in and of itself (though when you know enough actors, it’s clear many of the best are born with an ability to feed off of the limelight). He switches accents, does impressions, collapses on cue, and drags himself across the ground like Leonardo DiCaprio in “The Wolf of Wall Street” (but with less comedic flair). He laughs, cries, charms, rages, and hits every other conceivable emotion in a short timespan, and Cumberbatch does so without losing the character’s core. He’s always Patrick, even when he’s beyond recognition, but this kind of big performance is safer than it may seem: Any choice can be the right choice because Patrick is in that extreme of a state.

Later is when he truly shines, but by the end of the first episode, there’s no way of telling what’s next. Without spoiling anything, it’s safe to say the form “Patrick Melrose” takes initially isn’t sustainable; even in a five-hour story, watching a well-off white guy throw money around to sustain his addiction would be exasperating. Thankfully, “Patrick Melrose” shifts gears. The second episode almost cuts out Cumberbatch entirely, as a much-needed extended flashback fills in the many gaps from Episode 1 (and gives Jennifer Jason Leigh time to shine). The third episode brings it all together and starts gaining forward momentum. That may be too little, too late for some, but anyone who’s already on board because of the leading lad should be more than willing to ride out the series supporting him.

Perhaps in its final hours, “Patrick Melrose” can grind out a few relevant points regarding the entitled characters it loves and skewers; the class system is clearly on the mind ofDavid Nicholls, the writer who adapted Edward St. Aubyn’s novels for the screen, yet a specific statement has yet to emerge. A limited series can’t only be about one man’s performance, even if the actor does his part to earn the responsibility.

Grade: B

“Patrick Melrose” premieres Saturday, May 12 at 9 p.m. ET on Showtime.

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