  Let $\pi$ be a genuine cuspidal representation of the metaplectic group of
rank $n$. We consider the theta lifts to the orthogonal group associated to a
quadratic space of dimension $2n+1$. We show a case of regularised Rallis inner
product formula that relates the pairing of theta lifts to the central value of
the Langlands $L$-function of $\pi$ twisted by a character. The bulk of this
article focuses on proving a case of regularised Siegel-Weil formula, on which
the Rallis inner product formula is based and whose proof is missing in the
literature.
