London International Mime Festival, Southbank Centre

No wonder the German acrobat Tobias Wegner gets a special credit for the original idea behind Leo – it is simple, yet brilliant. The set, a bare room with a hanging lightbulb, is turned through 90 degrees, and the action on it is filmed and simultaneously played back in the vertical position. In this way, the stage becomes a split screen, one side showing a film in the normal perspective, the other highlighting the artistry of the performer, who is doing everything sideways. Or is he?

It is a neat visual trick, and an instant brain-teaser; in comparing the two sides, the audience must ask what is real, what comes first and which way is up. Performer William Bonnet plays with our expectations. How will he put his suitcase on the floor? Catch his hat? Drink water from a bottle? Cleverly, he uses and defies gravity, standing on one finger, spinning up the wall and punk-dancing upside down. Displaying incredible control and spatial awareness, his consummate acrobatics are always part of the story-making.

But this is more than a monkey puzzle: no sooner is the convention established than Bonnet and director Daniel Brière seek to push its creative limits and enter the surreal. In one lovely section, Leo adeptly draws his own room set, with chairs, table, bottle of wine, goldfish bowl and cat, and interacts with them. This develops into an animated film sequence, and the underwater ballet where he encounters sharks and whales to music from Swan Lake is delightfully absurd.

For a solo performer, the show is a mite too long (the Frank Sinatra sequence could be jettisoned), but nevertheless Leo goes further than ingenious entertainment. It spins its visual trickery with uncanny magic, distorting dimension. Suddenly we are looking down on Leo, trapped in a box, and there’s a real sense of fear and oppression. When film, set and human skill are in perfect synchronisation, this is intensely stimulating and sophisticated theatre.

Set on a sloping plane (plan in French), Plan B is also about gravity – our relationship with it, and impossible striving to escape it. But in contrast to Leo’s idiomatic struggle, its style is choreographed, almost balletic, with the circus skills part of the art rather than a diversion. Directors Aurélien Bory and Phil Soltanoff have conceived a full-scale vision in which lighting and technical effects work together seamlessly to throw the performers and the set into detailed relief.

To an electronic pulse, four young men in sharp, slim-fitting black suits slide over the tilting stage like a human waterfall, twisting, tumbling and adding their own variations. There’s a stealthy, Spider-man feel as they turn to scale the slope, which is by turn friend and foe, a pleasure-provider or a problem to be solved.

The slope buys time for the acrobats, unravelling their movements, highlighting the mechanics and the meaning. It offers up windows, spaces and ledges, which they explore like special forces agents, playing across the different levels with some exquisitely timed juggling sequences and hilarious climbing. Sometimes allies, at other times rivals, they are both a sinuous corps de ballet and exciting soloists. Who else could make bouncing balls against a wall a thing of poetry?

But when the horizontal filming method used in Leo comes into play, and the performers launch into a finale of set pieces, it feels slightly conventional compared with what has gone before. A kung fu sequence set to 1970s funk music is an undoubted crowd-pleaser, but little more. What we appreciate more is the weaving of great physical skills into an inventive, imaginative work that is much more than the sum of its parts.

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