  The Disjoint Paths Problem asks, given a graph $G$ and a set of pairs of
terminals $(s_{1},t_{1}),\ldots,(s_{k},t_{k})$, whether there is a collection
of $k$ pairwise vertex-disjoint paths linking $s_{i}$ and $t_{i}$, for
$i=1,\ldots,k.$ In their $f(k)\cdot n^{3}$ algorithm for this problem,
Robertson and Seymour introduced the irrelevant vertex technique according to
which in every instance of treewidth greater than $g(k)$ there is an
"irrelevant" vertex whose removal creates an equivalent instance of the
problem. This fact is based on the celebrated Unique Linkage Theorem, whose -
very technical - proof gives a function $g(k)$ that is responsible for an
immense parameter dependence in the running time of the algorithm. In this
paper we give a new and self-contained proof of this result that strongly
exploits the combinatorial properties of planar graphs and achieves
$g(k)=O(k^{3/2}\cdot 2^{k}).$ Our bound is radically better than the bounds
known for general graphs.
