  Movies of prominences obtained by space instruments e.g. the Solar Optical
Telescope (SOT) aboard the {\it Hinode} satellite and the Interface Region
Imaging Spectrograph (IRIS) with high temporal and spatial resolution revealed
the tremendous dynamical nature of prominences. { Knots of plasma belonging to
prominences} appear to travel along both vertical and horizontal thread-like
loops, with highly dynamical nature.
  The aim of the paper is to reconstruct the 3D shape of a helical prominence
observed over two and a half hours by IRIS.
  From the IRIS \ion{Mg}{ii} k spectra we compute Doppler shifts of the plasma
inside the prominence and from the slit-jaw images (SJI){ we derive the
transverse field in the plane of the sky. Finally we obtain the velocity vector
field of the knots in 3D.
  We reconstruct the real trajectories of nine knots travelling along ellipses.
  The spiral-like structure of the prominence observed in the plane of the sky
is mainly due to the projection effect of long arches of threads (up to 8 $
\times 10^4$ km). Knots run along more or less horizontal threads with
velocities reaching 65 km s$^{-1}$. The dominant driving force is the gas
pressure.
